Simple, hot-restartable Ruby service

super_simple_service.rb

require 'socket'

class SuperSimpleService
  attr_reader :bind_address, :bind_port

  def initialize
    @bind_address = 'localhost'
    @bind_port = 12345
  end

  def run
    if ENV['SOCKET_FD']
      @socket_server = TCPServer.for_fd(ENV['SOCKET_FD'].to_i)
      kill_parent
    else
      @socket_server = TCPServer.new(bind_address, bind_port)
    end

    @socket_server.autoclose = false
    @socket_server.close_on_exec = false

    setup_signal_traps

    loop do
      begin
        client_socket = @socket_server.accept # blocks until a new connection is made
        @connection_active = true
        client_socket.puts "Hello World!"
        client_socket.close
      ensure
        @connection_active = false
      end
    end
  end

  def setup_signal_traps
    trap('USR1') { hot_restart }
    trap('TERM') { graceful_shutdown }
  end

  def hot_restart
    fork do
      # :close_others ensures that open file descriptors are inherited by the new process
      exec("SOCKET_FD=#{@socket_server.fileno} ruby super_simple_service.rb", close_others: false)
    end
  end

  def graceful_shutdown
    @socket_server.close # Stop listening for new connections
    sleep 0.1 while @connection_active # Wait for active connection to complete

    Process.exit(0)
  end

  def kill_parent
    parent_process_id = Process.ppid
    Process.kill('TERM', parent_process_id)
  end
end

SuperSimpleService.new.run